This document provides an overview of hydrology and floodplain analysis. It discusses the hydrologic cycle and key concepts in hydrology like precipitation, infiltration, evaporation and transpiration. Atmospheric processes that produce precipitation like humidity, pressure, temperature and atmospheric circulation are explained. Measurement techniques for rainfall, streamflow, infiltration and evaporation are also summarized. The role of watershed characteristics in determining how rainfall produces runoff and the hydrograph is then outlined.

1. CEVE 412
Dr. Phil Bedient
Jan 2012
Hydrology and Floodplain Analysis,
Chapter 1

2. Hydrology
The study of the occurrence, circulation,
storage and distribution of surface and
ground water on the earth.
Areas of focus:
Hydrologic cycle
Fluid dynamics
Hydrodynamics
Water resource engineering
Water quality
Contaminant transport

3. The Hydrologic Cycle
Continuous process in which water is evaporated from water surfaces
and oceans, moves in land and precipitation is produced

4. The Hydrologic Cycle
Precipitation (P) – Rainfall, snow, etc.
Evaporation (E) – conversion of water to water vapor from a water
surface
Transpiration (T) – loss of water vapor through plant tissue and leaves
Infiltration (F) – water entering the soil system, function of soil
moisture, soil type
Ground water (G) – flows in
porous media in the subsurface
Runoff (R) – Overland flow,
portion of precipitation that does
not infiltrate

5. History
Water resource projects dating as far back as
4000 BC
Dam built across the Nile
First systemic flow measurement in U.S. in 1888
by USGS
1930s-1950s saw a boom in hydrologic knowledge
in US
Post 1950, scientists gained a greater
understanding of the effects of urbanization in
regards to hydrology
Computer advances have allowed for modeling of
complex hydrologic and hydraulic problems

6. Hydrology and Floodplain Analysis, Chapter 1.2

7. The Atmosphere
Atmosphere is a major hydrologic link
between oceans and continents and
facilitates the movement of water on the
earth
Major parameters
1. Atmospheric Pressure
2. Humidity
3. Precipitation

8. Atmospheric Pressure
Pressure = weight of air / unit area
Average Pressure at sea level (units)
1 atmosphere
1013 millibars (mb)
14.7 psi
760 mm-Hg

9. Ideal Gas Law
Describes behavior of gas under different
conditions
PV = nRT
P = Pressure
V = Volume
n = moles of gas
R = ideal gas constant
T = Temperature (Kelvin)

10. Gas Law and Atmosphere
Pressure and Temperature are
directly related at constant density
Temperature and Air Density (n/V) are
inversely related
Decrease in temperature increases
density
Affects movement of air masses
High pressure moves toward low pressure

11. Atmospheric Circulation
Fueled by solar energy
Uneven heating of the Earth
Concentrated at the equator
Warm air (low pressure)
travels upwards from the
equator and then towards the poles
Air shifts direction due to the Coriolis Force

12. Coriolis Force
Maintains angular
momentum
Mass of air wants to maintain
same speed, so it must
speed up as it leaves
equator, or slow down as it
moves towards equator
○ Point at the equator moves
faster than point near the pole
Causes air masses to “turn right” in northern
hemisphere, “turn left” in southern

13. Atmospheric Circulation
Coriolis effect creates westerlies, winds that
blow west to east in the northern hemisphere
Drive major weather systems in the U.S.

14. Air Masses and Fronts
Air Masses - large bodies of air with fairly
consistent temperature and humidity
gradients in horizontal direction
High Pressure System = Cold Weather
Low Pressure System = Warm Weather
Fronts are the boundaries
between two air masses

15. Humidity
Measure of amount of water vapor in
atmosphere
Specific Humidity - the mass of water vapor in
a unit mass of moist air at a given temperature
Relative Humidity – ratio of (air’s actual water vapor
content) to (amount of water vapor at saturation for
that temperature)
As air is lifted, it cools
Cool air “holds” less water
Eventually cools to the point that relative humidity is
saturated, and water vapor is condensed to liquid

16. Moisture Relationships
Specific Humidity (q)
q = (0.622*e) / (P - 0.378*e)
Vapor Pressure (e) – partial pressure exerted
by water vapor
e = (ρw*R*T) / (0.622)
○ ρw = vapor density or absolute humidity (g/cm3)
○ R = dry air gas constant
○ T = temperature (Kelvin)
Dew Point Temperature (Td) – temperature
that an air mass with constant pressure and
moisture content becomes saturated

17. Atmospheric Stability
Adiabatic Lapse Rate (ALR)
Rate of temperature change of air parcel with
change in elevation
Dry ALR = 9.8 oC/km
Rate varies with
moisture conditions
Causes stable or
unstable atmospheric
conditions

18. Cloud Types
Cirrus – feathery or fibrous clouds
Stratus – layered clouds
Cumulus – towering, puffy clouds
Alto – middle-level clouds
Nimbus – rain clouds
Cumulonimbus – thunderstorm clouds

19. Clouds

20. Precipitation
Condensed water vapor that falls to earth
Occurs when air parcel reaches saturation
i.e. the Dew Point Temperature is reached
Heat must be removed from moist air to allow
for condensation
Latent Heat
Major energy source
for storm systems

21. Precipitation Formation
Requires the following:
1. Moisture source
2. Lifting and resultant cooling
3. Phase change occurs with
condensation onto small
nuclei in the air
○ Range from 0.1 u – 10 u
○ Come from ocean salt, dust, etc
4. Droplets grow large enough to overcome drag
and evaporation

22. Lifting Mechanisms
Precipitation often classified by vertical lifting
Convective – Intense heating of the ground expansion and
vertical rise of air
Cyclonic – Movement of large air-mass systems (warm/cold fronts)
Orographic – Mechanical lifting of moist air masses over the
windward side of mountain ranges

23. Thunderstorms
Thunderstorms
Heavy rainfall, thunder, lightning, hail
Result from strong vertical movements or
warm, moist air
Generally occur due to
instability caused by:
○ Low-pressure systems
○ Surface heating
○ Forced ascent over
mountains

24. Thunderstorm Stages
Cumulus Stage
Moist air rises, cools and condenses into cumulus
clouds and continues to rise and condense
○ Updraft
Mature Stage
Rain begins to fall
Surrounding dry air is drawn into storm,
evaporates some drops and cools the air
○ Denser, cold air descends (downdraft) and creates
cool gusts of wind at ground level

25. Thunderstorm Stages (cont)
Dissipating Stage
When the updraft is cutoff
Rate of precipitation decreases
Downdrafts die-off
Clouds dissolve

26. Hurricanes
Intense cyclonic storms
Form over tropical oceans
○ Have localized names
Hurricane (N. Americe)
Cyclone (India)
Typhoon (East Asia)
Baguio (China Sea)
Energy comes from the condensation of very
warm, humid, tropical air
Categorized by the Saffir-Simpson Hurricane
Windscale

27. Saffir-Simpson Wind Scale
Category Wind Speed
(mph)
Extent of
Damage
Damage Description
Tropical Storm 39 – 73 Minor Some flooding
1 74 – 95 Minimal Limited damage, unanchored
mobile homes, trees
2 96 – 110 Moderate Some roof, door and window
damage
3 111 – 130 Extensive Some structural damage to
residences and utility buildings
4 131 – 155 Extreme Extensive curtainwall failures,
complete roof failures, all
signs blown down
5 156+ Catastrohpic Complete roof failure and
some complete building
failures

28. Hydrology and Floodplain Analysis, Chapter 1.3

29. Precipitation Trends
Varies geographically
Greater near coasts
Greater on windward side of mountains
○ Western side in US
Varies from season
to season

30. Rainfall Measurement
Why measure rainfall?
Water resource planning (annual)
○ California Water Project supplies water to
Southern California from Northern California
Urban drainage (hourly)
○ Reduce localized flooding
○ Need intensity and duration of rainfall
○ Spatial variation inside watershed

31. Point Measurement
Rainfall gage networks
Maintained by NWS, USGS or
local organizations
Typical gauge design
Methods of representation
Accumulated total rainfall
○ “Cumulative mass curve”
Rainfall Intensity vs. time
○ “Hyetograph”

32. Areal Precipitation
The average depth of precipitation over a
specific area (watershed)
Use point measurements to determine avg.
Three Methods
Arithmetic Mean
Thiessen Polygon Method
Isohyetal Method

33. Arithmetic Mean
Takes arithmetic mean of rainfalls from
available gages
Not accurate for large areas with variable
distribution
Only works if gages are uniformly distributed

34. Thiessen Polygon Method
Areal weighting of rainfall for each gage
Series of polygons created by lines
connecting eat gauge and perpendicular
bisectors
Uses ratio of polygon area to total area of interest
Most widely used method

35. Isohyetal Method
Draw contours of equal precipitation based
on gauge data
Uses area between each contour
Needs an extensive gauge network
Most accurate method

36. Next-Generation Radar (NEXRAD)
Allows for measurement of rainfall rates
and cumulative totals
Aided flood prediction
Specs
10-cm wave length
Records
○ Reflectivity
○ Radial Velocity
○ Spectrum width

37. Hydrology and Floodplain Analysis, Chapter 1.4-1.5

38. The Watershed
Def: Contiguous area that drains to an
outlet, specifically in regards to
precipitation
Basic hydrologic unit within which all
measurements, calculations and
predictions are made

39. Water Balance
I – Q = (dS/dt)
I = Inflow (L3/t)
Q = outflow (L3/t)
dS/dt = change in storage (L3/t)
Volume out of watershed =
(flow rate)*(time) OR
(depth)*(watershed area)

40. Water Balance
P – R – G – E – T = ∆S
P = Precipitation
R = Surface Runoff
G = Groundwater Flow
E = Evaporation
T = Transpiration
∆S = Change in Storage
Water balance for each area is different
Characteristics of the area alter how water
leaves watershed or basin

41. 1. Rainfall intensity and
duration
2. Size, Slope, Shape, Soil,
Storage
3. Channel morphology
4. Location of
Developments
5. Land use and cover
6. Soil type
7. Percent impervious
Divide
Floodplain
Reservoir
Natural
stream
Urban
Concrete
channel
Parameters that Affect Response in a
Watershed
Floodplain

42. Rainfall Runoff
Want to develop relationship of rainfall
minus losses vs. runoff for flood control
Allows hydrologists to determine flood conditions
based upon rainfall totals

43. Rainfall Runoff
Rational Method
Simplest rainfall-runoff formulas
QP = CiA
○ QP = peak flow (cfs)
○ C = runoff coefficient, varies with land use
○ i = rainfall intensity (in/hr) for a duration equal to
time of concentration (tc)
○ tc = time for a wave of water to propagate from the
most distant point of a watershed to the outlet
○ A = area of watershed (acres)

44. Hydrology and Floodplain Analysis, Chapter 1.6-1.7

45. Hydrographs
Plot of flow rate vs. time
Measured at a given stream
cross section
Mainly used to describe
stream flow response
from rainfall
Watershed characteristics
affect the shape
i.e. urbanization

46. Time Area Histogram
Computes hydrograph response for a
watershed
Breaks water shed into
distinct areas (Ai), have
equal travel time to outlet
Uses rainfall periods (Pi)
Rainfall from P1 in A2
reaches the outlet at the
same time at P2 in A1

47. Hydrographs – Broken Down
Typically characterized by:
Base Flow
Rising Limb
○ Increase in flow
Crest Segment
○ Peak flow rate
Recession Curve
○ Decrease in flow
Inflection Point
○ Point where direct runoff ends

48. Hydrograph Analysis
Total storm hydrograph is made up of Base
Flow and Direct runoff
Base Flow
Comes from ground water in absence of rainfall
Relatively small in urban environments
Direct Runoff (DRO)
Discharge caused by rainfall after infiltration losses
have been subtracted
“Rainfall Excess”

49. Hydrograph Analysis
How to separate Base Flow from DRO
Recession curves
○ qt = q0e-kt
q0 = specified initial discharge
qt = discharge at a later time, t
k = recession constant
Create these for each area of interest
More of an art than a science

50. Hydrograph Analysis
Hydrograph peak
Occurs when all areas contribute flow to the outlet
Dependent on watershed geography, storm
intensity/duration
Developed area
○ Higher, quicker peak flow
Natural, wooded area
○ Lower, slower peak

51. Hydrograph Analysis
Response to rainfall
Duration of rainfall is often shorter than time base
of hydrograph
Peak flow does NOT
correspond to peak
rainfall
Volume of water under hydrograph should
equal volume of precipitation minus base flow
and loses

52. Hydrograph Analysis
Infiltration and response
Rainfall (i) lose to infiltration (f) depends on
○ Soil Moisture Storage (SD)
○ Field Capacity (F)
Amount of water in a soil after gravity has drained it
If i < f
○ No overland runoff; all rainfall infiltrates
If i > f
○ Overland flow occurs
ϕ Index is simplest infiltration method
○ (gross precipitation) – (observed surface runoff)

53. Hydrograph Analysis
Infiltration and response
Horton Infiltration Method
○ When rainfall rate > infiltration rate, water infiltrates
at a rate that decreases
ϕ Index is simplest infiltration method
○ (gross precipitation) – (observed surface runoff)
○ Often underestimates losses at beginning
INSERT FIGURE 1-28 HERE

54. Hydrograph Analysis
Net vs. Gross rainfall
Gross Rainfall = depression storage +
evaporation + infiltration + surface runoff
Rainfall Excess = DRO = gross rainfall –
infiltration + depression storage

55. Hydrology and Floodplain Analysis, Chapter 1.8

56. Measurement Process
1. Sensing rainfall
Transforms intensity to measurement
2. Recording the data
3. Transmitting to central location
4. Translating data
5. Editing or checking for errors
6. Storing in database
7. Retrieving for further use

57. Measurement Devices
Barometer
Atmospheric pressure
Psychrometer
Relative Humidity
Gages
Precipitation
Radar
Rainfall rates

58. Evaporation Measurement
Evaporation is a major path of water lose
Rates vary location to location
Important to know rates for large-scale water
resource projects
Use instruments to measure rate in area
Class A pan

59. Infiltration Measurement
Small Scale: Ring Infiltrometer
2 ft. diameter ring driven into soil
Water is dumped into ring
Rate of infiltration is measured as water level drops
Large Scale: actual measurements
(Gross Rainfall) – (Direct Runoff from hydrograph)

60. Streamflow Measurement
What’s measured
Stage
○ Water elevation above
datum
○ Floating or bubbling gages
Use rating curves to
determine discharge
USGS must locate these at sites that are
accessible and have flow rates that relate to
depth

61. Rating Curves
Relate stage to flow rate at a cross section
Must be developed to determine discharge
Site specific
Created through actual measurements of
velocity at different stages
Use procedure that takes velocity measurements
at different depths at different parts of the
section